It is well known that photoconductive materials such as Cu.sub.2 O, CuI, ZnO, ZnS, ZnSe, CdS, Se-Te, CdSe, CdTe, PbS, Sb.sub.2 S.sub.3, In.sub.2 Te.sub.3, GeS, GeSe, Tl.sub.2 S and the like are dispersed in thermosetting resins such as epoxy resin, unsaturated polyester resin and the like, or thermoplastic resins such as vinylchloride-vinylacetate copolymer, polyvinylchloride, polyvinylacetate, cellulose acetate, cellulose nitrate, polyacrylate, polyvinylalcohol, polyvinylbutyral and the like to form photoconductive layers for electrophotography. Where an electrostatic latent image is produced using the photoconductive layer, electrostatic potential difference between the light portion and the dark portion of the latent image is extremely reduced under the conditions of high temperature and high humidity, thereby making it very difficult to form distinct images. It is thought that this phenomenon occurs due to the fact that the photoconductive layer absorbs moisture under the conditions of high temperature and high humidity and loses the characteristics of a photoconductive layer.
Referring to cadmium sulfide as a photoconductive particle, it is activated through doping by copper in the production thereof and, in this case, since the metallic salts added are present in the neighborhood of the surface of cadmium sulfide crystal or attached to the outer surface of the crystal, they should be removed by washing. However, such metallic salts cannot be completely removed by simple washing and a very small amount of the salt attaches to the outer surface of the cadmium sulfide crystalline particle. It is considered that the reduction of the characteristics of the photoconductive layer is caused by the interaction of the absorption of moisture by such salt's ion and the increase in electric conductivity as the result of the absorption of moisture.
As a matter of fact, it has been confirmed from the measurement of the amount of remaining ion that the conventional cadmium sulfide shows high electrical conductivity. Since the amount of remaining ion can not directly be measured, there is usually employed for the purpose a method in which the remaining ion is extracted from cadmium sulfide under a certain condition and electric conductivity of the extract so obtained is measured. According to this method, electric conductivity of cadmium sulfide is measured and the result is as follows.
For instance, 10 g of CdS is dispersed in 100 ml of pure water treated with ion exchange resin (electric conductivity below 1.0 .mu..OMEGA./cm, 20.degree. C.) and after the dispersion is boiled for two minutes, it is percolated to separate CdS. The thus-obtained liquid extract is cooled and the electric conductivity thereof is determined at the reduced temperature of 20.degree. C., which shows electric conductivity of the order of 30 to 50 .mu..OMEGA./cm, but not below 30 .mu..OMEGA./cm at all.
According to the conventional washing method, there can be obtained only CdS containing the remaining ion which shows electric conductivity of more than 30 .mu..OMEGA./cm by the above measurement. Thus, the resistance and the electrostatic voltage contrast between light and dark portions of the photoconductive layer are reduced at high temperature and the photoconductive properties with respect to humidity (resistivity to humidity) are reduced. Particularly, where the amount of remaining ion exceeds 50 .mu..OMEGA./cm by the above measurement, high electric conductivity due to the remaining ion appears, thereby reducing the contrast even though CdS is maintained at room temperature in the silica gel atmosphere for a week.
This phenomenon occurs where the electrostatic latent image is formed directly on a photoconductive layer and also where the electrostatic latent image is formed on an insulating layer provided on the photoconductive layer, thereby reducing the contrast of the latent image. Particularly, the photosensitive member in which an insulating layer is provided on the photoconductive layer and which is required for excellent capability to trap electric charges because the trapped electric charge in the photoconductive layer affects directly the latent images, is liable to lose such capability to trap electric charges at high temperature and high humidity, which results in low contrast.